The present invention relates to a numerical control system which controls the speed of rotation of the mandrel of a center lathe.
Numerical control systems are known which position a movable part of a machine tool on the basis of an instruction program containing the indication of discrete points of the path of the movable part, these being connected by rectilinear segments or by circular arcs. Interpolating means forming part of the numerical control system enable the numerical control of continuous type of the movable part to be effected on the basis of the coordinates of the discrete points.
In the application of these known systems to the numerical control of a lathe, it is therefore possible to execute turning operations by prescribing longitudinal dimensions and diameters of points of discontinuity in the desired profile of the workpiece, which are connected by rectilinear portions or circular arcs, and by moreover prescribing the speed of displacement of the tool along the path and the speed of rotation of the lathe mandrel.
However, while the speed of displacement of the tool along the desired path is influenced by considerations of a technological nature and with the required degree of finish, and therefore depends directly on decisions which are the concern of the programmer, the speed of rotation of the mandrel, on the other hand, is influenced by the cutting speed it is desired to employ and which also depends on the diameter of the workpiece at each point. In order to determine the program of the value of the speed of rotation of the mandrel which is to be prescribed, the programmer must therefore first decide the cutting rate desired in relation to the tool used, the material being machined and the depth of cut, and calculate from this the speed of rotation of the mandrel by dividing the cutting rate by the circumference of the work at that point.
This calculation is relatively simple and quick to carry out in the case where the workpiece is constituted by a succession of cylindrical zones such as to require the computation of a small number of speeds of rotation of the mandrel. The problem is complicated, however, when the workpiece comprises zones with a continuous variation in the diameter, for example conical or frustoconical zones or zones obtained by revolution from curves constituted, for instance, by a succession of circular arcs. In such zones, there must be prescribed a speed of rotation of the mandrel which is calculated as a ratio between the maximum acceptable cutting rate and the circumference at the point of the zone having the maximum diameter, with considerable extension of the machining times. If it is desired to maintain an optimum machining rate to reduce the machining times as much as possible, it is necessary to draw up a program specifying point by point the speed of rotation of the mandrel along the zone with a continuous variation in diameter, with consequence increase in the length and preparation time of the program, increase of the probability of errors and, in the end, relinquishing of the advantages offered by numerical control systems.
It would therefore be desirable to achieve the numerical control in such manner that the program governing the same contains, rather than an indication of the speed of rotation of the mandrel, an indication of the desired cutting rate.
Moreover, in the application of such numerical control systems of prior art to turning machining operations, the datum specifying the diameter is translated by the numerical control system into a position of the tool slide with respect to a selected reference plane such that the distance of the cutting edge of the tool used from the axis of rotation of the workpiece is equal to the desired radius, whereby the information relating to the actual diameter is lost. When the tool is replaced by another tool during the machining process, the position of the tool slide no longer corresponds to the required position of the cutting edge. Moreover, it may happen that the new tool has its cutting edge displaced longitudinally from the position of the cutting edge in the preceding tool. Both these factors give rise to errors both of longitudinal dimension and of diameter.
Although it is possible to take account of the different lengths and different longitudinal positions of the cutting edges of different tools by introducing artificial diameter and dimension data in which there are included the differences in length and dimension, it would be desirable to be able to formulate the programs with instructions containing the real data of the problem, with the addition of instructions specifying divergences of length or dimension in changing from one tool to another.